The invention relates to a transfer material detecting device which is useful in an image forming apparatus having a transfer material carrier, such as a color copier, or a color printer, and more particularly to a transfer material detecting device which detects a state where a transfer material stuck as a result of a jam remains held by a transfer material carrier. Such a transfer material detecting device is necessary for preventing a transfer material from being newly transported under a state where a previous transfer material remains carried by a transfer material carrier.
As a transfer drum of this kind, conventionally, the following technique is known.
(J01) Technique shown in FIGS. 14 and 15: PA0 Technique disclosed in Japanese Patent Publication No. SHO. 58-11965:
Referring to FIG. 15, a cylindrical photosensitive portion 02 is disposed at the center of the axial direction of a photosensitive drum 01. A photosensitive drum gear 03 is disposed at one end (the left end in FIG. 15) in the axial direction.
A toner image formed in the cylindrical photosensitive portion 02 of the photosensitive drum 01 is transferred to a transfer material carried by a transfer drum 06.
The transfer drum 06 shown in FIGS. 14 and 15 comprises a front drum 07 made of aluminum, a rear drum 08, and a drum tie plate 09 which connects the drums 07 and 08 with each other. Cylindrical members 011 and 012 for supporting a bearing are fixed to the center portions of the drums 07 and 08, respectively. The cylindrical members 011 and 012 are supported by a shaft member 013 via bearings (not shown). The shaft member 013 elongates outward from the drums 07 and 08 (see FIG. 14) so as to be supported by a frame (not shown) of an image recording apparatus.
A transfer film 014 (see FIG. 15) is stretched between the drums 07 and 08 so as to form a cylindrical shape. A transfer material (not shown) such as a recording sheet can be sucked to the cylindrical transfer film 014 by static electricity.
Referring to FIGS. 14 and 15, a ring-like driven gear (transfer drum gear) 016 is attached to the left end of the front drum 07 of the transfer drum 06. The transfer drum gear 016 engages with the photosensitive drum gear 03 of the photosensitive drum 01. A ring-like rubber belt 017 is disposed in the right side of the transfer drum gear 016. As shown in FIG. 15, the rubber belt 017 makes contact with the cylindrical face of the photosensitive drum 01 so that the distance (distance DDS, see FIG. 15) between the surface of the photosensitive portion 02 and that of the transfer film 014 is held at a predetermined value.
The rear drum 08 is not provided with a transfer drum gear. A rubber belt 018 (see FIG. 14) similar to the ring-like rubber belt 017 is disposed on the rear drum 08.
In the thus configured transfer drum, when a new image forming operation is started without removing a transfer material which is stuck on the transfer material carrier in a jam, a next transfer material is transported to the transfer material carrier on which the previous transfer material remains, thereby again causing a jam.
Therefore, the image forming operation must be started after ascertaining that no transfer material is held on the transfer material carrier.
As a technique for detecting that a transfer material exists at a predetermined position, conventionally, the following technique is known.
The publication describes a method in which existence of a transfer material on a transfer material carrier is detected by using a transfer material sensor of the reflected light detection type disposed in a moving path of a transfer material carrier. Generally, a transfer material is higher in optical reflectivity than the transfer material carrier. Therefore, a transfer material can be detected by the sensor of the reflected light detection type.
However, the transfer material is not disposed in the whole of the periphery of the transfer material carrier. Therefore, in the case where only one transfer material sensor is disposed in the periphery of the transfer material carrier, even when a transfer material is held on the transfer material carrier, the occasion when the transfer material does not exist at the position opposing the transfer material sensor sometimes arises.
On this occasion, even when the material is held on the transfer material carrier, the transfer material sensor cannot detect the transfer material. Therefore, in the case where only one transfer material sensor is used, the transfer material carrier must be rotated in order to surely ascertain whether a transfer material is held on the transfer material carrier or not.
When a transfer material is to be detected while rotating the transfer material carrier, there is a problem as discussed below. In a configuration wherein, as shown in FIGS. 14 and 15, the transfer film (transfer material carrier) 014 is supported in a cylindrical shape by the pair of drums 07 and 08 and the drum tie plate 09, the amount of light reflected from the drum tie plate 09 is not obviously distinct from that of light reflected from the transfer material, and hence the detection signal from the transfer material sensor is unstable.
FIG. 16 is a diagram showing the output signal of the transfer material sensor. In FIG. 16, the area defined by a indicates a detection signal for light reflected from the drum tie plate, and the succeeding area defined by b is a transfer material detection signal. FIG. 16 shows the signals obtained in the case where the transfer material exists. If there is no transfer material, the signal in the area b of FIG. 16 has a low level.
As seen from FIG. 16, in the region of the drum tie plate 09 (see FIG. 14), it is difficult to judge whether a transfer material exists or not.
In other words, the afore-mentioned method has the problems as discussed below.
In an image forming apparatus wherein a transfer material holding member which is hardly distinguished from a transfer material moves along the same path as the transfer material, it is impossible to surely detect existence of the transfer material.
In view of the above-mentioned circumstances, it is an object of the invention to detect the existence of a transfer material on a transfer material carrier rapidly and surely.
Next, the invention which has been accomplished in order to meet the afore-mentioned and other objects will be described. In order to facilitate the understanding of the relationships between the components of the invention and those of embodiments described later, the components of the invention are accompanied by parenthesized reference numerals of the corresponding elements of the embodiments. The reason why the invention will be described in correspondence with the reference numerals of the embodiments described later is to facilitate the understanding of the invention, and it is not intended to restrict the scope of the invention to the embodiments.